Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus

["Andreas Färber" <afaerber@suse.de>]

Am 16.02.2016 um 13:35 schrieb Markus Armbruster:
> Igor Mammedov <imammedo@redhat.com> writes:
> 
>> On Mon, 15 Feb 2016 20:43:41 +0100
>> Markus Armbruster <armbru@redhat.com> wrote:
>>
>>> Igor Mammedov <imammedo@redhat.com> writes:
>>>
>>>> it will allow mgmt to query present and possible to hotplug CPUs
>>>> it is required from a target platform that wish to support
>>>> command to set board specific MachineClass.possible_cpus() hook,
>>>> which will return a list of possible CPUs with options
>>>> that would be needed for hotplugging possible CPUs.
>>>>
>>>> For RFC there are:
>>>>    'arch_id': 'int' - mandatory unique CPU number,
>>>>                       for x86 it's APIC ID for ARM it's MPIDR
>>>>    'type': 'str' - CPU object type for usage with device_add
>>>>
>>>> and a set of optional fields that would allows mgmt tools
>>>> to know at what granularity and where a new CPU could be
>>>> hotplugged;
>>>> [node],[socket],[core],[thread]
>>>> Hopefully that should cover needs for CPU hotplug porposes for
>>>> magor targets and we can extend structure in future adding
>>>> more fields if it will be needed.
>>>>
>>>> also for present CPUs there is a 'cpu_link' field which
>>>> would allow mgmt inspect whatever object/abstraction
>>>> the target platform considers as CPU object.
>>>>
>>>> For RFC purposes implements only for x86 target so far.  
>>>
>>> Adding ad hoc queries as we go won't scale.  Could this be solved by a
>>> generic introspection interface?
>> Do you mean generic QOM introspection?
> 
> Possibly, but I don't want to prematurely limit the conversation to QOM
> introspection.
> 
>> Using QOM we could have '/cpus' container and create QOM links
>> for exiting (populated links) and possible (empty links) CPUs.
>> However in that case link's name will need have a special format
>> that will convey an information necessary for mgmt to hotplug
>> a CPU object, at least:
>>   - where: [node],[socket],[core],[thread] options
>>   - optionally what CPU object to use with device_add command
> 
> Encoding information in names feels wrong.
> 
>> Another approach to do QOM introspection would be to model hierarchy 
>> of objects like node/socket/core..., That's what Andreas
>> worked on. Only it still suffers the same issue as above
>> wrt introspection and hotplug, One can pre-create empty
>> [nodes][sockets[cores]] containers at startup but then
>> leaf nodes that could be hotplugged would be a links anyway
>> and then again we need to give them special formatted names
>> (not well documented at that mgmt could make sense of).
>> That hierarchy would need to become stable ABI once
>> mgmt will start using it and QOM tree is quite unstable
>> now for that. For some targets it involves creating dummy
>> containers like node/socket/core for x86 where just modeling
>> a thread is sufficient.
> 
> I acknowledge your concern regarding QOM tree stability.  We have QOM
> introspection commands since 1.2.  They make the QOM tree part of the
> external interface, but we've never spelled out which parts of it (if
> any) are ABI.  Until we do, parts become de facto ABI by being used in
> anger.  As a result, we don't know something's ABI until it breaks.
> 
> Andreas, do you have an opinion on proper use of QOM by external
> software?

This is absolutely untrue, there have been ABI rules in place and I held
a presentation covering them in 2012...

Andreas

> 
>> The similar but a bit more abstract approach was suggested
>> by David https://lists.gnu.org/archive/html/qemu-ppc/2016-02/msg00000.html
> 
> Cc'ing him.  If I understand the high-level idea correctly, David
> proposes to have an abstract type cpu-package with generic properties.
> Its concrete subtypes are composed of whatever components make up the
> hot-pluggable unit.
> 
> Management software can then use the generic properties to deal with hot
> plug without having to know about the concrete subtypes, at least to
> some useful degree.
> 
> Similarly, the generic properties suffice for implementing generic
> high-level interfaces like -smp.
> 
> David, is that a fair summary?
> 
> Naturally, we need a way to introspect available subtypes of cpu-package
> to answer questions like what concrete types can actually be plugged
> into this board.
> 
> This could be an instance of the generic QOM introspection question
> "what can plug into this socket"?  Unfortunately, I don't know enough
> QOM to put that into more concrete terms.  Andreas, Paolo, can you help
> out?
> 
>> Benefit of dedicated CPU hotplug focused QMP command is that
>> it can be quite abstract to suite most targets and not depend
>> on how a target models CPUs internally and still provide
>> information needed for hotplugging a CPU object.
>> That way we can split efforts on how we model/refactor CPUs
>> internally and how mgmt would work with them using
>> -device/device_add.
> 
> CPUs might be special enough to warrant special commands.  Nevertheless,
> non-special solutions should be at least explored.  That's what we're
> doing here.
> 


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